Hitherto, a heat-sensitive recording material which utilizes a coloring reaction of a colorless or pale-colored basic dye and a color developing material and provides a color image by contacting both coloring materials by the action of heat is well known as disclosed in, for example, U.S. Pat. No. 3,539,375. Recently, with the improvement of a heat-sensitive recording machine such as a heat-sensitive facsimile, a heat-sensitive printer, etc., using heat-sensitive recording materials, the recording speed of the heat-recording system is greatly increased. With the increase of the recording speed in these recording machines or hardware fields, heat-sensitive recording materials used for the purposes are, as a matter of course, required to greatly improve the recording sensitivity, and many attempts have been proposed in regarding to the recording sensitivity. However, since the improvement of the recording sensitivity encounters additional faults such as the reduction of whiteness of recording layer, etc., a satisfactory result is not always obtained. In such a circumstance, the inventors have made various investigations on improving the recording sensitivity of heat-sensitive recording materials without reducing the whiteness, etc.
In particular, various attempts have hitherto been made for improving the recording sensitivity by using a heat-fusible material together with a combination of a basic dye and a color developing material, and as the result of various investigations by greatly increasing the range of the investigation of the heat-fusible materials, the inventors have found that by using each of the compounds shown by general formulae (I) to (VI) as illustrated below, a very good effect of improving the recording sensitivity is obtained without reducing the whiteness as compared with the case of using a fatty acid amide, etc., such as stearic acid amide, palmitic acid amide, etc., and have succeeded in attaining this invention based on the discovery.